Graphite films are classified into: graphite films (also referred to as “natural graphite sheets”) produced by an expanding method; and graphite films (also referred to as “fired polymer-based graphite film”) produced by polymer thermal decomposition.
Fired Polymer-based graphite films are used as heat dissipation components for dissipating heat from components that generate heat, e.g., for semiconductor elements that are mounted in various electronic or electric apparatuses such as computers. As an example of the fired polymer-based graphite film, a graphite film having an excellent mechanical strength and having flexibility is disclosed in Patent Literature 1. According to Patent Literature 1, such a graphite film is obtained by raising the temperature of a polymer film of 75 μm in thickness up to 1000° C. in a nitrogen gas, heating a carbonized film thus obtained up to 3000° C. in an atmosphere of argon, and then subjecting a graphitized film thus obtained to a rolling process.
In addition, graphite films have the characteristics of having a lower tearing strength, as compared to typical plastic films and other films. Particularly, in the case of the fired polymer-based graphite films, their molecular structures are such that the molecules of graphite are more uniformly oriented in parallel to its planar direction. Thus, the fired polymer-based graphite films have the tendency to show low resistance to shearing stress, i.e., the tendency to show low tearing strength (average tearing force).
Patent Literature 2 discloses a method for reinforcing a pulling strength of a flexible graphite sheet (equivalent to a natural graphite sheet). According to this method, a flexible graphite sheet (natural graphite sheet) and a thin film of thermoplastic polymer having a given softening temperature are laminated to each other as they are supplied between temperature-controlled two calender rolls.
Further, heat dissipation components are used to dissipate heat from components that generate heat, e.g., for semiconductor elements that are mounted in various electronic or electric apparatuses such as computers. When a graphite film is used for large products among the heat dissipation components, the graphite film is preferably a long and large-area graphite film wound in roll form and made from a polymer film. Studies for producing such a graphite film have been made.
For example, there has been proposed a method of heat-treating a 30 m wide and 250 mm long polymer film wound around a carbonaceous cylindrical inner core having an outer diameter of 150 mm. This method makes it possible to obtain a long and large-area graphite film whose cylindrical hysteresis can easily be extended (Patent Literature 3).